--check does not exit with status code 1 when input file is stdin

--check doe not exit with status code 1 if the input file is stdin, despite what --help says.

Honestly, this behavior is preferable for our usecase, as we can already tell if the input is formatted correctly by the presence of the output diff, and can use the status code for more fatal errors.

$ rustfmt --version
rustfmt 1.4.38-stable (fe5b13d 2022-05-18)
$ rustfmt --help
...
        --check         Run in 'check' mode. Exits with 0 if input is
                        formatted correctly. Exits with 1 and prints a diff if
                        formatting is required.
...
$ rustfmt --check < server.rs
Diff in <stdin> at line 12:
 impl Drop for Server {
     fn drop(&mut self) {
         if !thread::panicking() {
-                self.handle.take().unwrap().join().unwrap();
+            self.handle.take().unwrap().join().unwrap();
         }
     }
 }
$ echo $?
0
$ rustfmt --check server.rs
Diff in /ssd/aosp-master/external/rust/crates/openssl/openssl/src/ssl/test/server.rs at line 12:
 impl Drop for Server {
     fn drop(&mut self) {
         if !thread::panicking() {
-                self.handle.take().unwrap().join().unwrap();
+            self.handle.take().unwrap().join().unwrap();
         }
     }
 }
$ echo $?
1

Reproduce the reported commands with rustfmt --check < server.rs and rustfmt --check server.rs, then compare their output and exit codes with the --help description. Done should be defined by the intended stdin behavior and corresponding check-mode documentation or implementation.